1. Field of the Invention
This invention relates generally geologic surveys and more particularly to an apparatus and method for producing acoustic signals in a marine environment.
2. Description of the Related Art
Seismic exploration is used to determine the presence of subsurface structures in earth formations that may contain useful materials such as hydrocarbons. During marine seismic operations, a service vessel often tows an acoustic source such as an array of air guns to generate high-powered acoustic signals. The acoustic energy transmitted by the acoustic source radiates generally downwardly into the sea floor and is partially reflected back towards the earth's surface by subsurface acoustic impedance boundaries, called reflectors, which may exist within the earth. The transducers (e.g, hydrophones), which are positioned at or near the water's surface, are used to detect this reflected acoustic energy. The transducers generate electrical signals proportional to the magnitude of the acoustic energy detected.
Typically, the acoustic source consists of an air gun array. A conventional air gun array can consist of a plurality of sub-arrays. Each sub-array includes several air guns that are placed in a pre-defined spatial relationship to one another. Auxiliary equipment, such as air gun controllers, is usually positioned immediately above these air guns. Additionally, supply lines such as air hoses, electrical power lines and communications run between the air guns and equipment on the service vessel. During operation, all the air guns of a sub array are activated simultaneously. Upon activation, the air guns release individual gas or air bubbles into the water. These bubbles produce acoustical energy that radiates into the water. The pulse characteristics, such as the frequency, bubble ratio and amplitude, of the overall pressure pulse produced by an air gun subarray is a function of the characteristics of the pressure pulses produced by the individual air guns and the physical arrangement of the air guns in that air gun subarray.
Certain seismic survey activities such as those targeting relatively deep subsurface formations or for reservoir mapping require a relatively powerful acoustic signal to obtain the necessary data. While an air gun can be configured to produce this powerful acoustical signal, the overall dimensions and weight of such an air gun can be substantial. It is generally undesirable to use heavy, bulky equipment in a shipboard environment because of factors such as limited storage capacity, lifting and handling requirements, and difficulties in deployment and retrieval.
Instead of a single air gun, a relatively powerful acoustic signal can be produced by an air gun cluster. That is, two or more relatively small air guns can be “clustered” to produce an acoustical signal similar to a signal produced by a much larger air gun. To achieve this affect, each gun in a cluster should be placed sufficiently close together so that their bubbles coalesce in the early stages of the bubble expansion. Conventional methods of clustering air guns include two or three air guns stacked in vertical alignment, i.e., perpendicular to the water surface. Another method uses a triangular relationship wherein a single air gun is superposed over two side-by-side air guns. It is also reported that air guns have been arranged in horizontal alignment.
These conventional methods of clustering air guns have a number of drawbacks. First, conventional clustering arrangements having air guns at different depths can produce asymmetric loadings. For example, the net reaction between two air guns can produce a damaging upward force on an air gun above the two air guns. Moreover, the vertically clustered air guns may need additional equipment to maintain their orientation during towing. More generally, the gun controllers and other auxiliary equipment used in conjunction with conventional air gun arrays can be exposed to the pressure pulse generated by the air gun array during use. Repeated exposure to the jarring affect of the air gun operation can shorten service life or lead to catastrophic failure of this equipment. Further, the supply lines associated with the air guns are frequently exposed to the wave action and contact with the water. This contact can cause, for example, failure in the joints making up the supply lines. In a different aspect, some conventional air gun arrays have profiles that induce substantial drag during towing.
The present invention addresses these and other drawbacks of conventional marine acoustic source systems.